Recovery of amines from coagulating baths



United States Patent RECOVERY OF AMINES FROM COAGULATING BATHS Charles S. McCandlish, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application November 18, 1953, Serial No. 392,985

8 Claims. or. 260-247) This invention relates to the spinning of viscose rayon, and relates more particularly to a method of removing and recovering amines from the acid coagulating bath of the viscose process when spinning amine-modified regenerated cellulose structures. This application is a continuation-impart of my copending application, Serial 'No. 240,803, filed August 7, 1951, now abandoned.

Recently it has been found desirable to spin viscose containing a small quantity of an amine to produce yarns and filaments having improved physical properties. In carrying out this process a portion of .the amine leaches out of the freshly formed filaments and remains dissolved in the bath as an amine sulfate salt. If allowed to accumulate its concentration will rise, which presents the problem of controlling the concentrations of amine salt in the bath to maintain satisfactory spinning conditions and uniform yarn production. For economical reasons, removal of the amine from the bath must be done without excessive loss of normal bath values and without the process becoming involved in complicated'chemical re actions.

It is an object of this invention to provide an effective and economical method of removing and recovering amines from viscose process acid-salt coagulating baths containing amine salts. Other objects of the invention will become apparent from the following disclosure and claims.

These objects are accomplished in accordance with this invention by passing coagulating bath liquor, containing sulfuric acid, inorganic salts and amine sulfate, through adsorbent carbon to substantially reduce the amine salt concentration and then, when the carbon has adsorbed a substantial amount of amine salt, stopping the bath flow therethrough and back-washing the carbon with hot water (70 to 100 C.) to obtain a first washing rich in sulfuric acid and inorganic salts and a second washing rich in amine salts. At the beginning of the hot water stripping the sulfuric acid and inorganic sulfate salts adsorbed on the carbon are preferentially released, very little amine salt being removed, and these first strippings can be returned to the bath system without further processing. As the acid, and inorganic sulfate salt concentration on the carbon falls, the rate of removal of amine sulfate rises rapidly and then gradually drops ofi again. The concentration of amine sulfate in the recovered back-wash water is in the neighborhood of 0.2 to 0.7% (calculated as amine) depending on the extent of stripping, rate of water flow, etc. From this solution the amine can be recovered by neutralizing and converting to the free amine. The amine solution can be concentrated thereafter in any of a number of ways, or the amine may be reused in the form of the dilute aqueous solution.

Before adsorption data on certain amines from water solutions were available showing that the free amine is much more strongly adsorbed than the amine sulfate, attempts were made to free amine sulfate from activated carbon by first neutralizing the adsorbed acid and freeing the amine by treatment with dilute caustic solution. No matter how this first step was followed up, not more than about half the adsorbed amine could be removed. However, by first washing the carbon with water to remove sulfuric acid and mineral salts, but without chemically changing the adsorbed amine sulfate, substantially 100% removal of the amine sulfate from the carbon was found possible. For practical and economic reasons it may be desirable to obtain an eflluent rich in amine salt by stopping the washing when only 60 to of the possible removal is accomplished. This simply reduces the carbon capacity, requiring a larger bed.

The following examples further illustrate the process of this invention:

EXAMPLE 1 A portion of used spinning bath was filtered through a bed of coal to minimize the amount of suspended sludge reaching the activated carbon. The filtrate, consisting essentially of about 9% sulfuric acid, 9.5% zinc sulfate, sodium sulfate and 0.046% cyclohexylamine sulfate (calculated as cyclohexylamine), was passed through a bed of 14 x 20 mesh activated carbon marketed by Pittsburgh Coke and Chemical Company as Type BPK. The bed of carbon was 46 /2 inches deep and was contained in a 6-inch diameter vertical column, wherein it was supported on a perforated lead disk covered with a plastic fabric composed of vinylidene chloride polymer. A similar lead structure covered with this fabric was placed on top of the bed to confine the carbon and keep it from floating in the bath.

The bath at 60 C. was fed upward through the bed at the rate of about 2.6 pounds per minute. Samples of bath entering and leaving the column were taken at frequent intervals and analyzed for cyclohexylamine sulfate. Under the conditions of test the inlet bath concentration of cyclohexyl-amine sulfate remained substantially constant throughout the run at about 0.046% (calculated as cyclohexylamine), while the eflluent bath was fairly constant in eyclohexylamine sulfate concentration for the first 12 hours at about 0.005% as cyclohexylamine and then gradually increased to a concentration of 0.046% (the concentration of the inlet bath) by the end of 22 hours total time. About 3,400 pounds of bath was passed through the adsorber and about 1.07 pounds of cyclohexylamine sulfate (calculated as cyclohexylamine) was adsorbed. The average cyclohexylamine sulfate content of the 3,400 pounds of bath leaving the adsorber was about 0.014% (calculated as cyclohexylamine).

After stopping the flow of bath through the adsorber and after allowing free drainage of mechanically held bath from the bed, water heated to a temperature of 90 C. was run down through the adsorber. The rate of water flow was maintained substantially constant at about 0.75 pound per minute and was continued over a period of 10 hours. Due to the low rate of flow and the relatively small diameter of the column, the temperature drop of the water passing through the adsorber was substantial, the outlet temperature being about 36 C. However, later work with adsorbers of larger diameter and similar height gave only a slight drop in temperature of not over 5 C. During the first half hour the removal of sulfuric acid, zinc sulfate and sodium sulfate was at a high rate, while the removal of cyclohexylamine sulfate was slight, e. g., the effluent averaged about 0.06% cyclohexylamine sulfate (calculated as cyclohexylamine). Thi portion of the washings was rich in desired bath values and was suitable to be put back in the bath system without processing other than adjusting the strength. The average cyclohexylamine sulfate concentration {in the eilluent wash water that "followed over a 9% hQ r period was about 0.24%, and about 97% of the cyclohexylamine sulfate adsorbed on the carbon was removed for reuse. Recovery of the amine is facilitated by stopping the washing while the concentration of amine is still relatively high in the efiiuent wash water. Thus the average concentration was about 0.65% after removal from the carbon and 0.52% after removal. The amineleft on the carbon is not lost but reduces the capacity of the adsorber in the next cycle, so an economic balance is involved.

The wash water contains, in addition to the cyciohexylamine sulfate, very small fractions of a percent of sulfuric acid, zinc sulfate and sodium sulfate. The cyclehexylamine sulfate may be converted to free cyclohexylamine by neutralization with lime or caustic. If lime is used for neutralization, calcium sulfate and Zinc hydrox ide will be precipitated and they can be removed easily by filtration. The filtrate from the lime neutralization may be distilled to obtain a concentrated solution containing over 40% cyclohexylamine, with elimination of most of the impurities in the waste discharge. Other methods are, of course, available for purification and/or concentration of this dilute solution of recovered amine.

EXAMPLE 2 A filtered spinning bath, similar to that of Example 1 but containing diethylamine sulfate instead of cyclohexylamine sulfate, having an amine salt concentration equivalent to 0.091% diethylamine, was passed through. a bed of 52 pound of activated carbon 44 inches deep contained in a 10-inch diameter column. The carbon was Type 56, 8 x 30 mesh, marketed by Pittsburgh Coke and Chemical Company. The bed was restrained between perforated lead disks covered with fabric composed of vinylidene chloride plastic. A total of 1420 pounds of bath was passed through the bed at a flow rate of about 0.55 pound per minute until 1.01 pounds of amine had been adsorbed after 43 hours, by which time the bed was no longer efficient for lowering the amine concentration of the bath. The average diethylamine sulfate content of the effluent bath was 0.020% (calculated as diethylamine).

The bed was now backwashed with 90 C. water at a flow rate of 1.7 pounds per minute for 5.1 hours. The exit wash water temperature was C. At first it contained only about 0.04% amine salt calculated as diethylamine, but after removal of sulfuric acid and mineral salts from the bed the concentration of diethylamine rose to 0.20%. Substantially 100% of the diethylamine adsorbed was removed in this way.

EXAMPLE 3 Under conditions similar to Example 2 except as noted, 4740 pounds of spinning bath containing 0.052% of N- methyl cyclohexylamine sulfate (calculated as the free amine) was passed through 52 pounds of SC- 8 X 30 activated carbon at a flow rate of 0.55 pound per minute in about 144 hours. The average amine sulfate content of the efiiuent bath was only 0.005% (calculated as N-methyl cyclohexylamine) and 2.23 pounds (as amine) had been adsorbed. The bed was nowhere near saturated, and equilibrium would not have been reached for at least 250 hours under these conditions.

The bed was backwashed with C. water at a tlow rate of 0.55 pound per minute for 24 hours. The exit water temperature was 85 C. and the concentration of amine salt in it rose from 0.06% to 0.19% (calculated as free amine) in the final stages. The recovery was 1.50 pounds of N-methyl cyclohexylamine or 67%.

Table I is a condensation of data for comparable conditions obtained from a large number of experiments with spinning baths containing a wide variety of amine salts. These data are presented in a form convenient for design purposes. For a given amine at a given concentration in the bath, the body of the table gives the pounds of amine per pounds of activated carbon adsorbed by the time the carbon'contains its equilibrium quantity at that hath concentration. From this value the amount of amine adsorbed by a bed containing any given weight of activated carbon is easily calculated, as is the quantity of bath which can be treated with the given bed in order to have a desired final concentration of amine. For flow rates of the order illustrated, which give adequate con tact between bath and bed, the length of time that a bed can be used until no more amine will be adsorbed may then be determined. Hence details of the type given in Examples l to 3 are readily derived from the comprehensive data of the table. Data for desorption of these amines are not given because all were readily recovered by washing the carbon with hot water.

Table l POUNDS OF AMINE ADSORBED PER 100 POUNDS 0F ACTI- VATED CARBON FOR SEVERAL CONCENTRATIONS Oi SPINNING BATHS Ethanolamine N, N Diethyl Ethyienedia- Morph oline. Piperazine. Ethylamine.

The maximum cycle is obtained by continuing to circulate the bath through an adsorber until no more amine salt is taken up by the activated carbon, but it will often be preferable to stop using an adsorber before the concentration of amine salt in the efiluent has increased appreoiably above the low value obtained during the first half of the complete cycle to saturation, asillustratcd in Example 3. In this way circulation of bath at a continuous rate through adsorbers used in rotation will remove amine salt at a continuous rate. The rate of circulation and the capacity of the adsorbers can then be arranged to remove amine at a rate equal to that at which it is being added in spinning, in order to keep the concentration in the spinning bath below the maximum of 0.05 92: mentioned. For similar reasons, instead of washing the activated carbon until substantially all of the adsorbed amine salt is removed, it is usually preferable to stop the desorption while the concentration of amine salt in the ez'fiuent wash water is relatively high, thereby reducing the expense of concentrating the effluent.

While each of the above changes, i. e., cutting short both the adsorption and the desorption parts of the cycle, acts to decrease the adsorber capacity for a given cycle, nevertheless it will generally be economically desirable to operate in that manner in actual practice. The activated carbon does not show any reduction in capacity from repeated use, so no ultimate loss of amine is incurred from not removing the amine salts as completely as possible in each cycle. If this were the only factor the ultimate recovery of amine would be substantially 100%. However, in actual practice bath losses and other processing looses reduce the recovery to 85-90%, depending on the amine concentration in the spinning bath.

The invention is not limited to the use of the types of activated carbon illustrated, for any good activated carbon may be used and the size may be varied somewhat from that given in the examples. The size of the bed and number of adsorbers will be determined by the throughput desired, and other considerations, in accordance with known engineering considerations.

Examples 1 to 3 and Table I have illustrated a large number of primary, secondary and tertiary monoamines and diamines which may be removed from aqueous acid baths, e. g., spinning baths, and recovered in accordance with this invention. These and similar amines to which the process is applicable are characterized by having a molecular weight of 60 to 156 and by consisting solely of one to two primary to tertiary amino groups linked to one to three organic groups each of Which groups contains from one to seven carbon atoms, may contain oxygen linkages and is hydrocarbon except for any oxygen atoms present. Amines of low molecular weight are unsuitable because of low adsorption from acid spinning baths. As shown by Table I, adsorption of ethylamine is too low for practical recovery. Amines of high molecular weight containing a group having a large number of carbon atoms are likewise unsuitable. The adsorption of n-decylarnine is negligible, for example. This is probably due to the surface activity efitect of long chain amines. Amines containing more than two amine groups, i. e., amines other than monoamines and diamines, are generally unsuitable. Tetraethylenepentamine is adsorbed as Well as aniline but is adsorbed so tenaciously that only about 5% was recovered by washing the activated carbon. Hexanrethylenetetramine is likewise unsuitable because carbon does not adsorb any significant quantity of it.

The invention is primarily concerned with the recovery of amines used as modifiers in the viscose spinning process, and amines suitable for this purpose are covered herein. All of the class of amines defined above are soluble to the extent of at least 0.1% in 6% sodium hydroxide and hence in the alkaline viscose solution. The invention enables efiicient recovery of high cost amines used in the spinning process with excellent control of the amine sulfate concentration in the coagulating bath, and thus insures good continuity of spinning and uniformly high quality products. Recovery of 85% or more of the excess amine salt dissolved in the bath by the simple direct methods above described are economically practicable and admirably fulfill the requirements of bath control.

As many different embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specfiic embodiments disclosed except to the extent defined in the appended claims.

What is claimed is:

l. A method of removing and recovering an amine salt from viscose process coagulating baths which comprises passing liquor from an aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of an amine, the said amine having a molecular weight of 60 to 156 and consisting of 1 to 2 primary to tertiary amino groups linked to 1 to 3 organic groups each said organic group being inert hydrocarbon and containing 1 to 7 carbon atoms, with the proviso that adjacent carbon atoms and adjacent carbon and hydrogen atoms may be separated by divalent oxygen, through activated carbon until the carbon has adsorbed a substantial amount of amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washingrich in amine salt.

2. A method of removing and recovering an amine salt from a viscose process cogulating bath which comprises passing liquor from an aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of an amine selected from the class consisting of benzyamine, di-n-butylamine, N,N-diethyl cyclohexylamine, N-methylcyclohexylamine, cylohexylamine, n amylamine, aniline, butylethanolamine, N methyl o methylcyclohexylamine, triethylamine, o-phenylenediamine, pyridine, diethylamine, triethanolamine, piperidine, ethylenediamine, hexamethylene diamine, ethanolamine, N,N-diethyl ethylenediamine, diethanolamine, morpholine, and piperazine, through activated carbon until the carbon has adsorbed a substantial amount of amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in amine salt.

3. The process of claim 1 wherein the first washing rich in acid and inorganic salt is recycled to a viscose process coagulating bath.

4. A method of removing and recovering amine salts from viscose process coagulating baths which comprises passing liquor from an aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of cyclohexylamine through activated carbon until the carbon has absorbed a substantial amount of the amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in the amine salt.

5. A method of removing and recovering amine salts from viscose process coagulating baths which comprises passing liquor from an aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of N-methylcyclohexylamine through activated carbon until the carbon has absorbed a substantial amount of the amine salt along with acid and inorganic salt, then Washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in the amine salt.

6. A method of removing and recovering amine salts from viscose process coagulating baths which comprises passing liquor from an aqueous coagulating bath containg surfuric acid, inorganic sulfate salt and the sulfate salt of N-methyl-o-methylcyclohexylamine through activated carbon until the carbon has adsorbed a substantial amount of the amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in the amine salt.

7. A method of removing and recovering amine salts from viscose process coagulating baths which comprises passing liquor from a aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of morpholine through activated carbon until the carbon has adsorbed a substantial amount of the amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in the amine salt.

8. A method of removing and recovering amine salts from viscose process coagulating baths which comprises passing liquor from 21 aqueous coagulating bath containing sulfuric acid, inorganic sulfate salt and the sulfate salt of hexamethylenediamine through activated carbon until the carbon has adsorbed a substantial amount of the amine salt along with acid and inorganic salt, then washing the carbon with hot water to recover a first washing rich in acid and inorganic salt, then continuing to wash the carbon with hot water to recover a second washing rich in the amine salt.

References Cited in the file of this patent Ruff: Chem. Abst, vol. 31, col. 1922 (7-8), 1936. 

1. A METHOD OF REMOVING AND RECOVERING AN AMINE SALT FRO VISCOSE PROCESS COAGULATING BATHS WHICH COMPRISES PASSING LIQUOR FROM AN AQUEOUS COAGULATING BATH CONTAINING SULFURIC ACID, INORGANIC SULFATE SALT AND THE SULFATE SALT OF AN AINE, THE SAID AMINE HAVING A MOLECULAR WEIGHT OF 60 TO 156 AND CONSISTING OF 1 TO 2 PRIMARY TO TERTIARY AMINO GROUPS LINKED TO 1 TO 3 ORGANIC GROUPS EACH SAID ORGANIC GROUP BEING INERT HYDROCARBON AND CONTAINING 1 TO 7 CARBON ATOMS, WITH THE PROVISO THAT ADJACENT CARBON ATOMS AND ADJACENT CARBON AND HYDROGEN ATOMS MAY BE SEPARATED BY DIVALENT OXYGEN, THROUGH ACTIVATED CARBON UNTIL THE CARBON HAS ABSORBED A SUBSTANTIAL AMOUNT OF AMINE SALT ALONG WITH ACID AND INORGANIC SALT, THEN WASHING THE CARBON WITH HOT WATER TO RECOVER A FIRST WASHING RICH IN ACID AND INORGANIC SALT, THEN CONTINUING TO WASH THE CARBON WITH HOT WATER TO RECOVER A SECOND WASHING RICH IN AMINE SALT. 